Chlorine is useful as a material for compounds such as vinyl chloride and phosgene. Chlorine is produced by known methods such as electrolysis of salt or catalytic oxidation of hydrogen chloride.
The electrolysis of salt consumes a large amount of electrical power and is thus energetically unfavorable. Further, since it gives caustic soda as a by-product, the balance between supply and demand of chlorine and caustic soda always matters.
On the other hand, the production of chlorine by the catalytic oxidation of hydrogen chloride was developed as a method for recovering hydrogen chloride by-produced in the production of vinyl chloride monomers or isocyanates. Since the by-product hydrogen chloride is used as a raw material, this process is very effective from the viewpoint of environmental load.
The production of chlorine by the oxidation of hydrogen chloride has three types: the electrolysis method, the gas-phase catalytic oxidation method and the non-catalytic oxidation method. The electrolysis method, proposed by UHDE GmbH in the 1960s, produces chlorine and hydrogen by the electrolysis of hydrogen chloride. Although this method has undergone various modifications, a problem still remains in that the method consumes a large amount of electrical power. The gas-phase contact oxidation method, also referred to as the Deacon process, was proposed in the 1860s as a method for producing chlorine from hydrogen chloride and oxygen. This reaction is an exothermic equilibrium reaction, and a lower reaction temperature favors the reaction. Known catalysts for this reaction are, for example, copper-based catalysts, chromium-based catalysts and ruthenium-based catalysts.
For example, known copper-based catalysts are catalysts in which copper chloride, an alkali metal chloride and lanthanoid such as didymium chloride are supported on a silica gel carrier having a specific surface area of not less than 200 m2/g and an average pore diameter of not less than 60 Å (Patent Literature 1), and catalysts which are prepared by impregnating copper, potassium and didymium in a silica gel having a specific surface area of 410 m2/g and a pore volume of 0.72 ml/g (Patent Literature 2). Although these catalysts are formed of inexpensive components, they have low reaction activity and should be brought to high temperatures to show sufficient activity. Since the Deacon process is an exothermic equilibrium reaction, the equilibrium conversion of hydrogen chloride is lowered with increasing temperature. Didymium is a mixture containing various rare earth elements. The mixture has a different composition depending on the place and time where it is mined. Accordingly, catalysts containing didymium show varied activity and are disadvantageous in obtaining stable performance.
For example, known chromium-based catalysts are catalysts in which chromia is supported on silicon oxide (Patent Literatures 3 and 4). Similarly to the copper-based catalysts, these catalysts have low reaction activity and are disfavored in obtaining a sufficient equilibrium conversion. Further, the major component chromium adds safety and health problems. Thus, the use of the catalysts constitutes a serious problem in terms of environmental loads.
For example, known ruthenium-based catalysts are supported metallic ruthenium catalysts, ruthenium oxide catalysts and ruthenium composite oxide catalysts (Patent Literatures 5 and 6). These catalysts show sufficient activity even at low temperatures. However, the main component ruthenium is expensive and thus should be recovered from the waste catalysts for reuse. Further, since ruthenium is a rare metal, a growth in demand easily leads to a price increase. Thus, the catalysts have problems in stable supply and costs.
A fluidized-bed process became widely known in the late 19th century. In this process, a reaction or a heat treatment is performed while solid particles are suspended by a fluid. The fluidized-bed process has been practiced also in the oxidation reaction of hydrogen chloride using a chromium-based catalyst. The fluidized-bed process requires that the solid particles show good fluidity continuously during the reaction. A number of studies have been then made with regard to particle properties, apparatus structures and operation conditions. In order that the solid particles keep good fluidity, the catalyst shape should be maintained during the reaction. A drastic change in catalyst shape due to, for example, abrasion or breakage during the reaction invites the scattering of the catalyst components and results in lower reaction activity. However, much is still unknown about the influences of these and other factors on the fluidity, and adequate researches have not been made.
Under this circumstance, the present applicant has found and disclosed that a catalyst having a particle diameter and a specific surface area in the specific ranges shows a small change in activity with time and hardly adheres together even when used in a fluidized bed (Patent Literatures 7 and 8).
However, the industrial chlorine production has still needed a chlorine production catalyst which allows for higher conversion to chlorine, has excellent catalyst life and shows excellent fluidity in the use in a fluidized bed.